1. Technical Field of the Invention
The present invention relates to a magnetic resonance imaging system capable of performing imaging known as interventional MRI (magnetic resonance imaging).
2. Related Art
Magnetic resonance imaging uses a radio frequency signal of a Larmor frequency to magnetically excite nuclear spins of an object to be imaged placed in a uniform static magnetic field. An MR signal resulting from the excitation is used to reconstruct an image.
Recently, the magnetic resonance imaging has been used in various medical fields. One such use is a treatment or examination to perform paracentesis with a puncture needle inserted from outside the body, operations of a catheter, surgery, and others with magnetic resonance imaging performed or under the state in which the magnetic resonance imaging can be performed. Such magnetic resonance imaging is called as interventional MRI, which has drawn much attention recently.
For the interventional MRI, it is significant that an operator recognizes, accurately and in real time, positions of devices such as a catheter inserted into a patient. Such techniques have currently been under development. It is dispensable for a safe and smooth surgery to accurately inform an operator of positions of the device in real time.
One known technique to detect the positions of the device such as a catheter is magnetic resonance imagining ordinarily for ordinal use. With this technique, the magnetic resonance imaging is performed ordinarily to directly image the position of a catheter to provide an operator with the position. This direct imaging of the catheter can be interpreted, from a physical point of view, as a technique of collecting the influence of the catheter on the magnetic fields as an indirect virtual image and making the operator recognize it on the image.
Another known technique is direct measurement of the position of a catheter. This measurement uses a minute RF reception (detection) coil disposed at the tip of the catheter. In the measurement, RF excitation for imaging is performed to cause an MR signal to be emanated from the vicinity of the RF reception coil. The MR signal is detected with magnetic gradients applied in the three-axis directions, respectively. Then the frequency of the MR signal is obtained, so that the tip position of the catheter is three-dimensionally calculated and displayed.
The measurement that adopts the RF reception coil is for example provided by a paper “1992 SMRM pp. 104, “Tracking of an Invasive Device within an MR imaging system,” C. L. Dumoulin, S. P. Souza, and R. D. Darrow.”
The measurement written in this paper employs a pulse sequence in which a region to be examined is first entirely excited an RF pulse, then a magnetic gradient in the X-channel is applied to detect an echo signal. The detected echo signal is subject to FFT (Fast Fourier Transform) processing, thus making it possible to calculate a shifted amount from a resonance frequency of a peak value in the frequency spectrum. Based on both the shifted amount and the intensity of the magnetic gradient, positional information in relation to the RF detection coil (the tip of the catheter) in the X-direction is calculated. Then, for each of the Y-channel and Z-channel, like the above, the RF excitation is carried out, a magnetic gradient is applied in the channel, and positional information abut the RF detection coil is calculated in each channel. Therefore, pieces of the three-dimensional positional information indicating the catheter tip can be obtained.
Further, another direct measurement that uses the RF reception coil is known, wherein a plurality of RF reception coils are disposed on a catheter in order to grasp and display an entire position of the catheter.
The interventional MRI involves, by way of example, a needling operation during which a device such as a puncture needle is gradually inserted into the body toward a target such as a tumor. Such an operation requires confirmation of needled states, i.e., whether or not the needle advances along a planned direction and/or whether or not the needle reaches a target.
To confirm those needled states, a conventional technique acquires sectional images each containing both of a needling start position obtained before paracentesis and a target. That is, during the needling operation, sections containing both the target and the puncture needle are imaged in sequence and used for monitoring needling states with reference to a reference section that has been imaged before the needling.
However, the foregoing conventional techniques have encountered various difficulties as follows.
First, there are problems concerning positional detection of a device such as a catheter.
A first problem is as follows. Since the positional detection according to the conventional ordinary imaging is based on the technique of imaging indirectly influence of a device on a magnetic field, it was difficult to provide an operator with an accurate position of the device tip. This is because the contour, positional shifts and/or intensity of the device on images are changed depending on imaging techniques; position, direction and material of a catheter; and imaging directions. Further, because a pulse sequence for imaging is used to recognize a device position, recognizing the position at a time instant always requires a period of time needed for one time of ordinary imaging. This results in a problem that resolution to display the position of the device tip is still lower.
A second problem derives from the conventional technique of attaching an RF reception coil on the tip of a device such as a catheter. In this technique, only one point (spot-like substance) indicating a coil position is depicted on images. This technique was unfavorable to the observation of the entire device. It was also difficult to grasp a direction along which the device is currently oriented and a direction along which the device should be needled from now on. Additionally, in this case, a slight spatial deviation of the catheter tip, which occurs due to hand shaking, makes it difficult to precisely recognize the tip spatial position.
A third problem relates to the technique of attaching a plurality of RF reception coils on the device. In this configuration, it is necessary to perform the positional detection for each of the plural RF reception coils. This makes the entire system complicated and it takes time to measure the positions of the coils.
Second, there are problems concerning monitoring needled states of a device such as a catheter.
A first drawback is concerned with a reference section (that is, an imaged section containing both a needling start position and a target) acquired before starting the paracentesis. The reference section is frequently shifted due to breathing and body motions of a patient. Such shifts become obstacles to a smooth needling along a scheduled path, requiring a great deal of time and work in the needling operation. This may cause trouble in treatment and examination.
A second drawback will be caused when an operator inserts a puncture needle in a direction different from the reference section. If such an insertion is done, the needle tip deviates from the reference section, thus failing to display the tip on images. Hence the conventional monitoring technique was not suitable for needling operations involving subtle operations.
A third problem arises due to an interval between a preoperative plan and an actual treatment. Normally, prior to the paracentesis, a needling plan (preoperative plan) that includes planning both of a start position for needling and a path for needling is done based on a reference image acquired beforehand. However, the planned position and path are obtained only on an image, but the planned ones differ in feeling from actual positions on the body surface of a patient to be needled. Hence, in performing the actual paracentesis, it takes much time for an operator to understand positional relationships about the needling start position and needling path based on the plan. This results in a lowered patient's throughput and frequently generates the situation where needling results in different ones from the planned needling start position and planned path.